From my travels around the US and other countries giving invited talks at universities, I know that seminar series can be scheduled for any weekday and are held at an impressive range of possible times of the day or evening.
I suppose a case can be made for why any particular day of the week is better for a particular department based on class schedules and other academic activities specific to that place, but in general:
Some places favor Monday because it is close to the weekend, making travel for visitors somewhat easier, but the week is still young and everyone is feeling energetic and ready to start off the work week by being invigorated by a stimulating talk on cutting-edge research.
Some places favor Tuesday because it isn't Monday, when people might not be quite ready to dive into the work week, but Tuesdays aren't too deep into the week that people have lost energy and focus.
Some places favor Wednesday, though it is hard to think of a good reason. Perhaps it is good to have a day in the middle to pause and do something different.
Some places favor Thursday because it is getting close to the weekend, perhaps facilitating travel for visitors who will have a 2-day visit, but it is not Friday.
Some places favor Friday because it makes travel easier for visitors and people are ready to wrap up the week by being invigorated by a stimulating talk on cutting-edge research.
In terms of time of day, mornings are probably the least likely time for a seminar series, but any time after (and including) noon seems to be fair game, up until about 6 pm (+/-).
Giving talks at noon is always a bit weird for the speaker because of all the food smells and food eating sounds that permeate the seminar room. In terms of attending seminar talks, however, I don't have a strong personal preference as long as a seminar is likely to be over in time for me to pick up my daughter from school at 5:30.
The poll below asks about what day of the week your department holds its most general seminar series involving outside speakers, whether or not this is your favorite/preferred day of the week.
9 hours ago